\documentclass[12pt]{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
  \vec{u} = \begin{bmatrix}
      3\\1
  \end{bmatrix}
\end{align*}

\begin{align*}
  \sigma_{\vec{u}} = \sqrt{\sum_{i=1}^N (u_i - \mu_u)^2}
\end{align*}
\end{document}
